Well since you wrote longrifle, then we're talking about a minimum 38" barrel, and a full stock. The Lyman plains style rifles are great guns, as are offerings from Dixie, and Pedersoli has other "longrifle" models with short (35") barrels. Sorry, though, (imho) they don't fit the historic term "longrifle" with a barrel under 38". IF that's the look you want, you just don't get it with a shorter barrel.
NOW, if you can, get the Blue Ridge in .54. That's the lightest configuraion. A .50 works fine, but it is a tad heavier. The rear sight is a buckhorn style. I cut off the "ears" on my rifle's rear sight, so all it was was a simple square piece of steel. THEN I swapped the front sight for a thin, silver, front sight blade (Track of The Wolf). You can't get the best out of the barrel with one of those steel front sights, so swap it out. It's only a few bucks, and a little tapping with a brass hammer, and a little filing.
You adjust the front sight to sight her in. File off a bit of the front sight slowly, to raise the point of aim. Use a punch and drift the sight a bit right or left to adjust windage. I use 70 grains of BP and a patched round ball, and sighted mine in 3" high at 25 yards. It was dead-on at 75, and about 1" low at 100.